Abstract

Anandamide was the first brain metabolite shown to act as a ligand of "central" CB1 cannabinoid receptors. Here we report that the endogenous cannabinoid potently and selectively inhibits the proliferation of human breast cancer cells in vitro. Anandamide dose-dependently inhibited the proliferation of MCF-7 and EFM-19 cells with IC50 values between 0.5 and 1.5 microM and 83-92% maximal inhibition at 5-10 microM. The proliferation of several other nonmammary tumoral cell lines was not affected by 10 microM anandamide. The anti-proliferative effect of anandamide was not due to toxicity or to apoptosis of cells but was accompanied by a reduction of cells in the S phase of the cell cycle. A stable analogue of anandamide (R)-methanandamide, another endogenous cannabinoid, 2-arachidonoylglycerol, and the synthetic cannabinoid HU-210 also inhibited EFM-19 cell proliferation, whereas arachidonic acid was much less effective. These cannabimimetic substances displaced the binding of the selective cannabinoid agonist [3H]CP 55, 940 to EFM-19 membranes with an order of potency identical to that observed for the inhibition of EFM-19 cell proliferation. Moreover, anandamide cytostatic effect was inhibited by the selective CB1 receptor antagonist SR 141716A. Cell proliferation was arrested by a prolactin mAb and enhanced by exogenous human prolactin, whose mitogenic action was reverted by very low (0.1-0.5 microM) doses of anandamide. Anandamide suppressed the levels of the long form of the prolactin receptor in both EFM-19 and MCF-7 cells, as well as a typical prolactin-induced response, i.e., the expression of the breast cancer cell susceptibility gene brca1. These data suggest that anandamide blocks human breast cancer cell proliferation through CB1-like receptor-mediated inhibition of endogenous prolactin action at the level of prolactin receptor.

A CB1-like cannabinoid receptor mediates anandamide effect on EFM-19 cell proliferation. (a) Dose-related effects of two cannabimimetic compounds, HU-210 and 2-arachidonoylglycerol, and of a non-CB1 agonist, palmitoylethanolamide, on EFM-19 cell proliferation, as compared with anandamide. (b) Effect of two different doses of the CB1 antagonist SR 141716A on the anti-proliferative action of anandamide (1 and 2.5 μM) and arachidonic acid (AA, 1 μM). (c) Displacement of [3H]CP 55,940 from EFM-19 cell membrane preparations by anandamide (AEA), HU-210, 2-arachidonoylglycerol, SR 141716A, and palmitoylethanolamide. In a and b, data are mean ± SD (n = 3) and are expressed as in Fig. a, c, and d. In c, data are expressed as percentages of [3H]CP 55,940 bound to membranes, are means of triplicates, and are representative of three distinct experiments. To avoid confusion, we do not show SD bars. Asterisks indicate statistically significant differences from data without SR 141617A. ∗, P < 0.05; ∗∗, P < 0.01. N.S., not significant.

Anandamide interferes with prolactin action. (a) Dose-related effect of prolactin mAb (Pierce) on EFM-19 cell proliferation in the presence or absence of 1 μM anandamide (AEA). (b) Effect of human (h) prolactin (50 ng/ml) on EFM-19 cell proliferation and its counteraction by low doses of anandamide with or without 0.5 μM SR 141716A. (c) Effect on the levels of the long form (100 kDa) of prolactin receptor of 3-day treatment of EFM-19 cells with anandamide (2.5 μM) in the absence (lanes B and E) or presence (lanes C and F) of SR 141716A (0.5 μM); lanes A and D are from untreated cells. (d) Effect on the levels of the brca1 gene product (220 kDa) of 3-day treatment of EFM-19 cells with anandamide (2.5 μM) in the absence (lane B) or presence (lane C) of SR141716A (0.5 μM). In a, the difference observed between the two sets of data was never statistically significant except for 0 μg/ml prolactin antibody. In a and b, data are mean ± SD (n = 3) and are expressed as described in Fig. a, c, and d. ∗, P < 0.05 vs. h-prolactin + [AEA] = 0; ∗∗, P < 0.05 vs. h-prolactin + [AEA] = 0.5 μM. In a, control experiments were performed by using BSA or a NO synthase III polyclonal antibody instead of prolactin mAb, with no effect on proliferation. In c and d, Western immunoblotting was performed with a monoclonal anti-prolactin receptor antibody (c, lanes A–C), polyclonal anti-phosphotyrosine antibody (c, lanes D–F), or a polyclonal anti-brca1 protein antibody (d, lanes A–C). Proteins immunoprecipitated with a monoclonal anti-prolactin receptor antibody or total proteins (50 μg) were used in c and d, respectively. Control experiments (not shown) did not exhibit the bands at 220 or 100 kDa and were carried out with: (i) no proteins, (ii) no first antibody, and (iii) using, as the first antibody, various antibodies other than the ones mentioned above. The mobility of molecular weight markers is shown. Data are representative of at least three separate experiments. Similar data were obtained with MCF-7 cells. Photographs were taken from films exposed with the enhanced chemiluminescence methodology.